text: An optical filter can be used to block out visible light and near-ultraviolet light. It is important to have a high transmittance within the solar-blind spectrum, but to strongly block the other wavelengths.
  Interference filters can pass 25% of the wanted rays, and reduce others by 1000 to 10,000 times. However they are unstable and have a narrow field of view.
  Absorption filters may only pass 10% of wanted UV, but can reject by a ratio of 10. They can have a wide field of view and are stable.